Most Helpful Girl
She is not slutty girl but you are lol.. you connected with her physically so of course she started to feel things for you.
Did you tell her you had a girlfriend at that time? I guess not so it is kind of your fault because you lied to both ladies.
Just tell your current girlfriend the truth so in case this girl tells her you won't come of as liar...
I think both ladies owe you an apology too and to be honest there's no way to "stop" it you gotta be a man and take resposibility for what you did :)
Most Helpful Guy
Quite honestly, you can't stop her from telling anyone assuming she can find a way to figure out who your girlfriend is (which is pretty easy nowadays).
All you can really do is damage control now that you are guilty of cheating. I'd tell the girlfriend first of the situation. If she hears it from an outside source first, you are pretty much screwed.
Another thing is that you'll want to emphasize it was a purely physical event and not emotional. Psychology experiments have shown that women are more likely to forgive physical cheating compared to emotional (men are opposite in that regard).
You should be prepared though to be dumped, dropped, and cursed at. This is simply consequences of an action made and there is a high chance this will happen from the nature of cheating.
Perhaps you can use this situation as a learning experience. Whether or not you continue in the relationship or get dumped and find another, DON'T cheat again! Remember this guilt and etch it into your soul. Remember the awkwardness and how you never want to have an experience like this again. Remember it all so you can prevent a repeat.
All that can be done is damage control and explaining the situation to your girl from your side.
(Note: I wouldn't consider a slutty girl considering you chose to initiate too. It isn't all her fault as you are both to blame. "Takes two to tango" as they say.)